C++反射那些事

对于C++反射，我关心的使用领域包括：模版元编程可以解决的那一部分编译时反射，运行时反射在实现IoC、ORM等中可以让程序员写得更惬意的语法糖衣、DSL，在类Rspec测试和Mock中所需要的对于被测程序非侵入式的信息获取和校验。

从 http://www.garret.ru/cppreflection/docs/reflect.html ，我们可以了解到在C++中达成反射的基本方法：

Approach	Advantages	Disadvantages
Parse debugging information	Program should not be changed It is possible to extract complete information about types used in the programs	Program was build with enabled debugging support and distributed with debugging information There are a lot of different formats of storing debug information, while there are some standards for formats of executable files (COFF, ELF,...) format of debugging information usually is more or less specific to the particular compiler
Special preprocessor which will parse C++ sources and build class descriptors	Program should not be changed	Alignment and offset of fields in classes, as well as virtual function table format and naming conventions are specific to each compiler, so it is not possible to create generic preprocessor which will produce type information directly. Instead of it, it should generate some C++ code, which should be later processed by normal C++ compiler. So build procedure becomes too complex. Parsing of C++ code is not trivial task. Also to be able to modify preprocessed code, this tool should fully handle standard C++ preprocessor directives and has to be called with the same options as normal compiler.
Create specialized compiler which will support reflection	This is most convenient case for the user - no extra steps during build are required to produce runtime type information. Compiler can generate efficient code for reflection methods	Creating own C++ compiler is very challenged task. Users will still prefer to use their own favorite tools (Visual C++, Borland,...). It is mostly because most of the modern compiler are not only compilers but IDE with builtin editor, debugger, profiler, wizards, class libraries....
Let programmer to provide this information himself	This is the most portable approach which can be used with any C++ compiler This approach is the simplest to implement With this approach it is possible to include additional type information (user defined attributes, for example transient)	Program has to be changed Programmer has to do more work There is a chance for programmer to make mistake describing class format

utensil/cpp_relection.md